The present invention generally relates to fasteners and threaded workpieces, and more particularly to affixing an internally threaded insert within a workpiece so that a threaded fastener may be made up to the workpiece utilizing the threads of the insert.
For a variety of reasons it is desirable to dispose a self-tapping sleeve within a workpiece. The self-tapping sleeve will typically have internal threads and will be utilized replacing the original threads in the workpiece. For example, if pre-existing threads of the workpiece are damaged, the damaged threads may be replaced with the threads of the insert. The inserts discussed herein are self-tapping, such that the insert may be driven into a bore of the workpiece cutting new threads in the bore as the insert is driven (i.e., the “bore threads”). The self-tapping inserts have external threads for making up into the bore threads and internal threads for receiving a fastener. The inventor herein is also the inventor of U.S. Pat. Nos. 7,819,613 and 8,439,617 which teach different embodiments of self-tapping inserts and methods for installing the inserts. The installation methods disclosed in the '613 and '617 patents are, to some degree, applicable to the installation of the presently disclosed invention and the '613 and '617 patents are specifically incorporated herein by this reference.
The most common use of self-tapping inserts is to provide replacement threads where the original threads of the workpiece have become damaged, and to stabilize the matrix material to prevent cracks from continuing or propagating. When the original threads become damaged, they can sometimes be repaired by chasing the damaged thread with a tap to restore the original thread shape. However, if the original thread shape cannot be restored by this measure, the thread must be replaced. One means of replacing the threads is to bore the hole to a larger diameter than the original thread diameter and to rethread the hole. However, a disadvantage of this procedure is that it requires a change in the fastener size from the original. If the equipment utilizes multiple fasteners of the original size, the different size fastener complicates maintenance and repair of the equipment because different tools are required, and correlating fasteners with the matching threads made more difficult. It is therefore desirable in some cases to be able to replace the original threads with threads of the same size so that the same size fastener may be utilized. In these cases, self-tapping inserts may be employed.
Self-tapping inserts are hardened steel cylinders, threaded on the exterior and, usually, in the interior. The interior thread diameter and pitch of the internal threads are those of the fastener to be installed.
The exterior of the self-tapping insert comprises a section which cuts new threads (the “cutting section”) and a section of threads which make up into the newly cut threads. The cutting section of the known self-tapping inserts is tapered and usually comprises three or more slots or holes, which interrupt the tapered threads, thereby forming teeth similar to those of a conventional thread tap. Some type of driving mechanism is used to drive the self-tapping insertion into a pilot hole in the base metal of the workpiece. Various driving mechanisms may be utilized for this purpose, and may include drive bolts or other devices which engage or grip the self-tapping insert and allow sufficient rotational force to be applied to drive the self-tapping insert into the workpiece. The driving mechanism typically drives the self-tapping insert into a pilot hole in the base metal. This pilot hole is usually made by drilling out the damaged threads to form a bore hole in the base metal. As the insert is turned, the teeth of the cutting section engage and remove the base metal until the insert is fully installed and flush with the exterior surface of the workpiece. The insert remains in place within the workpiece by an interference fit between the newly cut threads in the workpiece and the exterior threads on the insert.
The known self-tapping inserts generally rely upon an interference fit between the newly cut threads and the external threads of the insert to prevent the insert from backing out of the base metal. The small metal chips generated by the cutting action of the insert assist the interference fit by wedging between the external threads of the insert and the new threads of the base metal. While this phenomenon is effective in preventing back-out of the insert from the base metal, it increases the torque requirements for installing the insert.
It is desirable that once the self-tapping insert is installed in the workpiece that it stays installed. In other words, it would be undesirable if the self-tapping insert were to unscrew from the workpiece as an internal fastener is either screwed into the insert or removed. One situation which can create such an undesired situation is where a fastener with damaged threads were installed into the insert. It is possible that defective/damaged threads will create so much friction (gall) as the bolt is rotated (in either installation or removal) that the insert will back out of the work piece.
For example, the self-tapping inserts of the inventor herein typically have exterior threads which are the reverse of the internal threads of the self-tapping insert, i.e., where the internal threads are a conventional right-hand thread, the cutting threads and exterior threads are typically left-hand threads. Ordinarily, this feature prevents the self-tapping insert from backing out when an internal fastener is removed. However, if a damaged internal fastener is utilized, there may be such friction generated that the self-tapping insert backs out as the fastener is installed. Thus, using a damaged fastener in a self-tapping insert can result in the unintended backing out of the insert from the workpiece.
There are other situations in which damaged fasteners have undesired results when utilized with self-tapping inserts. For example, in a number of applications for self-tapping inserts parts are secured to a workpiece by fasteners made up into the insert, where the parts are installed and removed often. Examples of such applications include molds in injection molding of plastics, dies in stamping presses, choks in steel mills, etc. While it is not a recommended practice to reuse fasteners, unfortunately it happens, particularly if the fasteners are an unusual size and/or are expensive. If a fastener is to be reused, a recommended procedure is for the operator to: (1) visually check the condition of the fastener; (2) visually check the condition of the threaded hole; (3) hand-install the fastener in the hole to double-check its condition and fit; and (4) use a torque wrench to accurately torque the fastener. However, in practice, this procedure is too frequently ignored. The operator is instructed to “get the product run going NOW.” The situation is compounded if the operator installs the fastener with an impact wrench to save time. The result, either immediately or over time, is damage to the threaded holes.
Thus, a need exists for a self-tapping insert which can repair damaged threads of a fastener.